Pressure cell press comprising a tray, and a method for manufacturing said tray

ABSTRACT

The present invention relates to a press of pressure cell type, a tray for use in a press of pressure cell type and a method. The tray comprises a tray frame, which defines a space for arranging a forming tool and/or a work-piece. Prestressing means are arranged on the external surface of the tray frame and induce a compressing prestress which acts in planes parallel to the plane of the tray. The tray frame presents a curvature along its entire circumference.

FIELD OF THE INVENTION

The present invention relates to a press of pressure cell type, a trayfor use in a press of pressure cell type and a method for manufacturingsuch a tray.

BACKGROUND ART

A press of pressure cell type generally comprises a force-absorbingpress body which defines a press chamber. In the upper part of the presschamber, a press plate and a diaphragm of rubber or another resilientmaterial are arranged, which together form a pressure cell. The pressurecell communicates with a source of pressure and expands when a pressuremedium is supplied. In the lower part of the press chamber, a structuralsupport or a tray is arranged, which comprises a bottom plate having atray frame. The tray supports a working tool or a forming tool, aworkpiece, a mat of rubber or another resilient material, covering theforming tool and the workpiece.

Presses of pressure cell type are used, among other things, when formingsheet-shaped blanks, for example sheets of steel or aluminum, for shortseries products within the aircraft industry and the motor industry. Thesheet is placed in the press in such a manner that one of its sidesfaces a forming tool. The resilient diaphragm is arranged on the otherside of the sheet. A closed space between the diaphragm and the pressplate located above the diaphragm constitutes the pressure cell and thisspace is filled during the forming process with a pressure medium. Bypumping additional pressure medium into the pressure cell, the pressureis increased in the pressure cell and the resilient diaphragm is pressedduring stretching against the sheet which, in its turn, is formed roundor in the forming tool. When the sheet completely fits to the tool, thepressure in the pressure cell is released and the diaphragm is removedfrom the sheet, after which the formed component can be taken out of thepress.

Another field in which presses of pressure cell type are used is woodcompaction when a workpiece of wood is exposed to high pressure, eitherin a forming tool or on its own. Reasons for compacting wood are, forexample, that it is desirable to increase the hardness of the wood,decrease the moisture content or to obtain a phase in pressureimpregnation.

In traditional presses of pressure cell type, use has been made of aforged tray, in which at least the short sides of the tray are providedintegrally with the bottom of a tray. The short sides and the radiustransition to the bottom of the tray have to be dimensioned in such amanner that they can manage high working pressures. This means that thetray becomes unnecessarily thick and heavy.

SE 452 436 discloses a press of pressure cell type which was developedwith the purpose of solving the above-mentioned problem. Said patentspecification discloses a press plant having a forged, cylindrical pressbody which defines a press chamber. A tray which supports a forming tooland a workpiece is inserted into the press chamber. A large annularsupport which is arranged round the press body is adapted to absorb loadbeing induced on the tray during a pressing operation. Each time thetray is to be taken out or inserted, the annular support has to beelevated in order to make the press chamber accessible. This is acomplicated and time-consuming method.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a press of pressurecell type which reduces the above-mentioned problems.

Another object of the invention is to provide a tray which, incomparison with prior-art technique, gives advantages when handling apress of pressure cell type.

These and other objects which will be evident from the followingdescription are achieved by a press of pressure cell type, a tray and amethod, which have the features indicated in the appended claims.

In the following specification, it should be understood that a “tray”according to the present invention means a device with the purpose ofincluding a forming tool and/or a workpiece. In the traditional sense,it may thus comprise walls and a bottom plate. However, it should alsocomprise an essentially annular configuration which is adapted to bearranged in a detachable manner on a separate bottom plate; for example,the tray can rest on a bottom plate belonging to a press body, or abottom plate which can be pushed in and pulled out of the press. In thedetachable variant, the tray is thus tubular and has a through aperturewhich is defined by an annular wall configuration or tray frame. Herein,annular means a shape that forms a closed path.

In the present application, terms describing position and direction,such as “vertical” and “horizontal” are used. In the application, theseterms are defined with respect to the tray arrangement. Consequently,the circumference of the tray arrangement runs horizontally, whereas itsheight has an extension vertically. In the application, it should alsobe understood that “over/upwards/above” and “under/downwards/below” aredefined with respect to the main direction of the pressing, i.e. so thata press plate is located above a diaphragm which, in its turn, islocated above a bottom plate. This means that vertically is defined asperpendicular to the press plate and horizontally as parallel to thepress plate. The plane of a tray is thus a horizontal plane. Theabove-mentioned definitions have been indicated for the sake of claritysince the press of pressure cell type can be inclined in differentmanners and, due to this fact, the relative directions can vary.

According to one aspect of the invention, a press of pressure cell typeis provided. The press comprises a force-absorbing press body whichencloses a press chamber, in which press chamber a tray is arranged. Thetray comprises a tray frame which defines a space for arranging aforming tool and/or a workpiece. According to the invention,prestressing means are arranged on the external surface of the trayframe and induce a compressing prestress which acts in planes parallelto the plane of the tray, i.e. essentially horizontal planes. The trayframe presents a curvature substantially along its entire circumference.

According to another aspect of the invention, a tray is provided for usein a press of pressure cell type. The tray comprises a tray frame,wherein prestressing means that induce a compressing prestress, whichacts in planes parallel to the plane of the tray, are arranged on theexternal surface of the tray frame. The tray frame presents a curvaturesubstantially along its entire circumference.

The present invention is thus based on the understanding thatconsiderable improvements regarding handling and time expenditure can beprovided by moving the force-absorbing function closer to the actualtray, without needing to use forged, thick trays. Consequently, theinvention does not apply the known principle of improving the techniqueby having external means, such as an annular support, which are arrangedoutside the press body in order to absorb forces which are induced inthe tray during the pressing. On the contrary, such external means areexcluded in the present invention by prestressing means instead beingintegrated with the tray. According to the invention, the tray is thusadapted to independently absorb or withstand load unlike the prior-artpress having the annular support. The prestressing means according tothe invention allow a considerable decrease of the thickness of the trayas regards short sides or ends and radius transitions compared with atraditionally forged tray. This means that the tray according to theinvention allows a greater working depth in relation to previously knownforged trays and also that aspects such as manufacturing and transportof the tray are improved.

In addition to the prestressing means, the curved shape of the trayframe also conduces to the tray being able to independently absorbradial load around the whole horizontal plane. According to at least oneembodiment the external surface of the tray frame is curved in itscircumferential direction, while the internal surface of the tray frame,i.e. the surface that defines said space, presents straight portions inthe circumferential direction. Alternatively, the internal surface ofthe tray frame may be curved in the circumferential direction, while theexternal surface of the tray frame presents straight portions in thecircumferential direction. According to at least another embodiment boththe internal surface and the external surface of the tray frame arecurved in the circumferential direction. Within the scope of the presentinvention it is thus possible to provide an independentlyforce-absorbing tray by providing at least one of said internal surfaceand said external surface curved in the circumferential direction of thetray frame.

The external surface of the tray frame has preferably circular shape.Alternatively, the internal surface of the tray frame has circularshape. Another possibility is that both the internal surface and theexternal surface have circular shape.

Instead of said circular shape the tray frame may have an elliptical oran oval shape. A further alternative is a shape of a super-ellipse, i.e.a closed curve which is something between an ellipse and a rectangle,having the equation |x/a|^(n)+|y/b|^(n=)1, where the exponent n>2.

Thanks to the fact that the at least one of said surfaces of the trayframe lacks straight portions in the circumferential direction, e.g. inaccordance with one of the above mentioned shapes, such stressconcentration that otherwise appear in the tray due to straight portionsis avoided. The prestressing means are adapted to induce a compressingprestress on the tray in planes which are parallel to the plane of thetray, i.e. planes which are perpendicular to the main direction of thepressing. Preferably, the prestressing means are arranged on theexternal circumferential surface of the tray, i.e. the external surfaceof the tray frame, in the form of wound prestressing elements. Saidplane of the tray lies preferably in parallel with a bottom plate onwhich a forming tool and/or a workpiece is intended to be arranged.

According to an advantageous embodiment of the press of pressure celltype, the tray comprises a number of plate-shaped lamellar means whichabut against one another. Each lamellar means is annular and has acentral through hole. The lamellar means which are plate-shaped are thusarranged on one another in different planes of the tray or planes of theplate and are arranged concentrically with the central holes. Aworkpiece, such as a metal sheet or a piece of wood, is intended to bemachined in the space that is mutually formed by the holes of theconcentric lamellar means.

Advantageously, the tray is prestressed in such a manner that eachlamellar means is individually prestressed. This is preferably providedby a prestressing element being arranged on each lamellar means. It hasturned out to be particularly advantageous to use and wind by means of aprestressing element that is band-shaped and has essentially the samewidth as the thickness of a lamellar means.

It is made clear by that mentioned above that the invention is alsobased on the understanding that, by dividing the tray arrangement intoseveral annular parts, the manufacturing and transport of a tray arefacilitated, and the handling is made easier when the press of pressurecell type is in operation. These parts or lamellar means can beassembled to a tray at the location where the press is to be used andcan also be dismounted individually for further transport or storage.The dismountability also has advantages when a press plant is inoperation, which will be made evident by the following description.

The tray can have a tool-holding function. In the lower portion of thespace, a forming tool can be arranged having a workpiece arrangedthereon. In the case of wood compaction, the forming tool can beexcluded.

As already mentioned, at least one preferred embodiment of the inventioncomprises the features that the tray is divisible because of the factthat it comprises lamellar means of the above-described type which aredismountably arranged on one another. In connection with a pressingoperation, the lowest lamellar means is preferably detachably arrangedon a bottom plate in the press chamber. A diaphragm support ispreferably arranged above the uppermost lamellar means, and a pressplate is, in its turn, arranged above the diaphragm support. The holesin the lamellar means thus together form a space which is defined by theinner wall of the internal lamellar means, the bottom plate and adiaphragm which is placed in the diaphragm support. Depending on, forinstance, working depth, one or more lamellar means can be arrangedbetween the lowest lamellar means and the diaphragm support.Alternatively, the tray comprises only one lamellar means.

Preferably, the divisible tray is arranged in the press chamber in sucha manner that the diaphragm support placed above the tray can be liftedin the direction towards the press plate. This allows a practicalinsertion and removal of the tray as will be described in the following.Actuating means, such as hydraulic pistons, are suitably adapted to liftthe diaphragm support (and possibly also one or more lamellar means). Atits upper portion, the inner diameter of the diaphragm supportessentially corresponds to the circumference or diameter of the pressplate and, due to this fact, the diaphragm support can be made toenclose the press plate when it is lifted upwards. It is convenient thatthe diaphragm support is so high that it encloses the press plate alsoin a non-lifted state so that satisfactory sealing is obtained duringpressing.

The diaphragm support is preferably formed as a lamellar means, which asregards its appearance is essentially similar to the lamellar meanscomprised in the tray, and is adapted to hold a diaphragm which forms apressure cell together with the press plate. Since the diaphragm isgenerally not removed or replaced as often as the workpiece is, it is anadvantage if it is not necessary to remove the diaphragm support fromthe press when a workpiece or forming tool is to be removed from thepress.

An advantage of the lifting function in the press chamber as describedabove is that the replacement of a workpiece or a forming tool isfacilitated. Instead of lifting a heavy annular support, which isarranged outside the press, relatively high up in order to obtain accessto the tray in the press chamber, it is thus sufficient to lift thediaphragm support so that a gap is provided (which does not exist when apressing operation is carried out), the underlying lamellar means beingeasy to remove in the direction of the main axis of the press chamber,since there is no friction against the diaphragm support. The directionof the main axis of the press chamber is in a horizontal plane. Analternative is that the diaphragm support and one or more lamellar meansare lifted up and the lamellar means which is/are positioned therebeloware taken out whereas the remaining ones are left in the press.Subsequently, the remaining lamellar means can be lowered (with orwithout the diaphragm support) by means of the actuating means to thebottom of the press chamber and, in a corresponding manner, theselamellar means can be lifted up to the intended position before apressing operation.

The internal lamellar means are advantageously loosely arranged on thebottom plate and on one another; however, a type of control elements arearranged in order to ensure correct placements. Due to the fact that theinternal structure of the press comprises lamellar means which areloosely arranged on one another, it is possible to easily take these outseparately or several at the same time.

The divisibility of the tray arrangement results in several advantagesin that the lamellar means can have several purposes; on the one hand,they may constitute a direct or indirect support for a working tool orforming tool on which for example, a metal sheet is to be shaped and, onthe other, they can support or fasten various parts which are active inthe press. For instance, a diaphragm which together with the press plateforms a pressure cell can be clamped between two lamellar means or theuppermost lamellar means and the press plate. Alternatively, thediaphragm can rest loosely against a shelf which protrudes from theuppermost lamellar means which corresponds to the above-describeddiaphragm support. A mat which is used to protect the diaphragm and isplaced below the same can be fastened between two lamellar means. In acorresponding manner, the metal sheet can be fastened with the aid ofsuitable means. Likewise, the metal sheet becomes easily accessibleafter a terminated pressing operation by lifting one or more lamellarmeans that lie above the metal sheet.

Due to the advantageous embodiment having prestressed lamellar meanswhich have internal surfaces, external surfaces, or both, that lackstraight portions in the circumferential direction of the lamellarmeans, no external force absorber is needed. The press structure cantherefore be made relatively open by the ends or any short sides of thepress chamber wall, i.e. the external sides of the lamellar means, beingaccessible with the aim of inserting and removing the internal lamellarmeans. In the assembled press, part of the internal lamellar means willpreferably protrude at the ends of the press beyond the actual pressbody.

The lamellar means which advantageously are used to form a tray arecurved, such as for instance circular or oval as regards their shape.Each annular lamellar means comprises a wall configuration, i.e. a trayframe which defines a central hole. On certain occasions, it can beadvantageous if the holes have an essentially rectangular or squarecross-section in the horizontal plane, e.g. depending on the shape ofthe forming tool. If the internal surface of the wall configuration iscurved, such as circular this is advantageously provided by means offilling blocks of resilient material, such as rubber, which are arrangedto make contact with said internal surface. The purpose of the fillingblocks is, among other things, to serve as support for a forming tool.If the forming tool is large enough, the filling blocks can be excluded.It is also suitable to use filling blocks in wood compaction since aworkpiece has the shape of a right-angled block. In addition, thepurpose of the filling blocks is to absorb and distribute forces andstress which are generated during a pressing operation. From the aboveit is thus clear that filling blocks are also useful when it isdesirable to substantially maintain the geometrical shape of the spaceand the aim is only to provide further support for the tool and a forcedistributing function. It is also to be noted that other cross-sectionsthan the above mentioned are possible, e.g. depending on the shape ofthe forming tool.

The lamellar means in the tray according to the invention can be giventhe desired shape by milling or cutting. Different types of cutting arepossible, a few examples being water cutting, plasma cutting and flamecutting. Those skilled in the art will realise that this is aconsiderably simpler process than forming the traditional compact trayby forging. There will also be a great simplification as regardstransport of the lamellar means which are each relatively light, incomparison with transport of trays according to prior-art technique.Preferably, the tray or the lamellar means are made of hot-rolled steelsheet which subsequently is easily given the desired shape. In thepresent invention, it has been found that it is suitable to use a sheetthickness of 80–200 mm, preferably 100–150 mm, especially 100–120 mm.

Due to the fact that the lamellar means are separate units which, bydegrees, together are to form a tray, manufacture of them can beaccelerated considerably. Thus, various lamellar means blanks can bemachined in the respective stations at the same time. A first lamellarmeans blank can be machined in a certain station and when this lamellarmeans blank has been moved on to a subsequent station for furthermachining, a second lamellar means blank can be machined at the sametime in said certain station. This parallel performing of differentmanufacturing steps thus turns out to be very beneficial. It is alsodistinctly easier to move a relatively thin lamellar means in comparisonwith a large traditional tray. Preferably, some stations can machineseveral lamellar means blanks simultaneously.

The lamellar means are easily transported to the location where thepress of pressure cell type is intended to be used and assembled insitu. It has been found that the tray structure according to theinvention having integrated prestressing means functions excellently attypical working pressures (such as 2000 bar) for presses of pressurecell type. Instead of making a large and heavy tray, it is possible todivide the structure into several plates which each weigh less and thusare easier to handle.

Although the tray according to the invention advantageously is used in apress chamber which is enclosed by a traditional, forged press body, ithas been found to be practical to make also the press body offorce-absorbing lamellar means, and because of this fact the main partof the press can be manufactured in the same way and is easy totransport in parts which are subsequently assembled at the locationwhere the press is to be used.

It is also possible to assemble each lamellar means from two or moreparts, which then by said winding of a band are connected to a coherentunit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, partly in cross-section, of a press of pressurecell type according to one embodiment of the present invention.

FIG. 2A shows the press of pressure cell type in cross-section along theline A—A in FIG. 1.

FIG. 2B is a top plan view of an internal lamellar means of the typeshown in FIG. 2A.

FIG. 2C shows a plan view of a lamellar means in FIG. 2A.

FIGS. 3A–3E are end views of different variants of presses of pressurecell type according to the invention.

FIGS. 4A–4B illustrate alternative geometrical shapes for a tray frame.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view (of the long side), partly in cross-section, of apress of pressure cell type 10 according to one embodiment of thepresent invention. A central portion of the press of pressure cell type10 is cut out of the Figure, an ordinary side view of the press beingshown to the left of the central portion and a side view incross-section of the press being shown to the right of the centralportion. The press of pressure cell type 10 is essentially made up ofplate-shaped lamellar means. A force-absorbing press body is formed ofexternal lamellar means 12 which are vertically arranged at a distancefrom one another. Each external lamellar means 12 has a central hole,the press body thus enclosing a press chamber in which the actualpressing operation takes place. An upper press plate 14 and a bottomplate 16 run through the central holes of the external lamellar means12. Between these plates, a diaphragm support 18 and an internalhorizontal lamellar means 20, which abut against one another, arearranged. The diaphragm support 18 is plate-shaped and annular and thushas a shape that essentially corresponds to the internal lamellar means20. The internal lamellar means 20 rests detachably on the bottom plate16, whereas the diaphragm support 18 is arranged so that it partlyencloses the press plate 14 (shown in the right part of the figure) withthe purpose of ensuring sufficient sealing.

The circumference of both the internal and the external lamellar means12, 20 (also the diaphragm support 18) is defined by a relativelynarrow, circumferential, external edge surface 22. A plurality of turnsof a band 24 of spring steel are wound round the external edge surface22 of the lamellar means 12, 20 and the diaphragm support 18, the band24 having a width which essentially corresponds to the thickness of alamellar means 12, 20 and a diaphragm support 18, respectively. Theheight of the band layer 24 of the lamellar means 12, 20 and of thediaphragm support 18 is about 100 mm and the layer can consist of onesingle long band or a plurality of joined pieces of band. When alamellar means 12, 20, is being manufactured, the band 24 is wound roundthe same under resistance so that a compressing prestress is permanentlyinduced in the lamellar means 12, 20. FIG. 1 shows that the trayaccording to the invention, i.e. the internal lamellar means 20, iswithout external support at the ends of the press of pressure cell type10 since the turns of the band 24 replace that function satisfactorily.For the same reason, the diaphragm support 18 does not have an externalsupport.

The right part in FIG. 1 is as mentioned a side view in cross-section ofthe press of pressure cell type 10.

The cross-section is made at the middle of the press, i.e. along themain axis of the press chamber. The right part of FIG. 1 clearly showsthat both the lamellar means 12, 20 and the diaphragm support 18 arewound with a band 24 on the respective external edge surface 22. Theturns of the band 24 of the internal tray-forming lamellar means 20 andthe diaphragm support 18 are, according to the invention, intended toessentially permanently limit the expansion of these, i.e. they shouldbe able to resist the forces that are formed in the press chamber. Theinternal lamellar means 20 is annular, which thus means that it definesan internal, open space 26, being comprised in the press chamber. Adiaphragm 28 is arranged in the open space of the diaphragm support 18.The diaphragm has a seal 30 against the press plate 14 and forms apressure cell with the same. When in operation, the pressure medium issupplied to the pressure cell in such a manner that the diaphragm 28expands. The open space 26 of the internal lamellar means 20, which isplaced below the diaphragm 18, is adapted to contain a forming tool orworking tool. A metal sheet which is to be pressed against the workingtool is arranged in a suitable manner above the working tool, thediaphragm 28 in connection with pressurisation being expanded and shapedon the working tool, which means that the metal sheet locatedtherebetween also is shaped on the working tool. In addition, the Figureshows a mat 32 which is arranged just below the diaphragm 28. The mat 32takes part in the shaping of the metal sheet and protects at the sametime the diaphragm 28 against wear and tear.

The external lamellar means 12 are, apart from the central hole, eachprovided with four circular apertures, two above and two below the hole.The apertures are adapted to receive coupling means. Through thecircular apertures in all the external lamellar means included in thepress body run coupling means 36 (two of which are shown), for example asteel rod having threaded ends. The press-body-forming lamellar means 12are kept at a distance from one another by the fact that round eachcoupling means 36, between the lamellar means 12, there are distancemeans 38 having a thickness that is as large as the desired distancebetween the lamellar means. The distance means 38 are made of arelatively rigid material and their inner diameter is larger than thatof the coupling means 36 at the same time as their external measures areessentially larger than the apertures arranged in the lamellar means 12.At the two external ends of the coupling means 36, outside therespective external lamellar means 12 which are included in the pressbody, there are stop devices 40 of which at least one has a fixing andclamping mechanism which is complementary to the coupling means 36. Inthe case when the coupling means comprises a rod being threaded at itsends, the attaching and stressing mechanism can comprise a washer and anut, the washer having external measures which are essentially largerthan the coupling apertures of the external lamellar means. The fourcoupling means 36 are thus tightened to a predetermined prestresscondition. This eliminates play and motion in the construction and atthe same time contributes to the structural stability of theconstruction as regards flexural rigidity, torsional rigidity andresistance to extension in all dimensions.

A typical operating pressure inside the shown press of pressure celltype is 2000 bar.

FIG. 2A shows the press of pressure cell type in cross-section along theline A—A in FIG. 1. The Figure shows that an external lamellar means 12is plate-shaped. The central through holes of the external lamellarmeans 12 are defined by an internal edge surface. The hole isessentially quadrangular, but without actual corners. The “cornerregions” are instead rounded and bend inwards into the wall so that alarger hole area is obtained. The radii of these inward bends are maderelatively large with the aim of minimising the stress concentrationthat arises in the corner regions.

The external lamellar means 12 is essentially quadrangular and hasrounded corners. The shape of the lamellar means 12 is adapted to theexpected thrust which arises in connection with the pressing. Thus, thematerial quantity or the distance between the internal and the externaledge surface is larger vertically than horizontally since the maindirection of pressing is vertical.

A plurality of turns of a band 24 of spring steel are wound round theexternal edge surface of the external lamellar means 12, the internallamellar means 20 and the diaphragm support 18 which are shown in FIG.2A, the band 24 having a width which essentially corresponds to thethickness of the respective lamellar means 12, 20 (or diaphragm support18). Each band can be one single long band or a plurality joined piecesof band.

FIG. 2A also shows that the internal lamellar means 20 and the diaphragmsupport 18 are not supported by an external sidewall or the like, butare surrounded by an empty space 50.

FIG. 2B is a partial top plan view of an internal lamellar means 20 ofthe type shown in FIG. 2A. Thus, it is shown that this lamellar means 20has the shape of a circle. Since the internal lamellar means 20 and thediaphragm support 18 are prestressed by the turns of the band 24, noexternal limiting means are required and therefore portions of thelamellar means 20 and the diaphragm support 18 can protrude from theends of the press body as shown in FIG. 1. Since the internaltray-forming lamellar means 20 protrudes, it is relatively easilyaccessible, which is time-saving when metal sheets are removed, toolsare replaced, diaphragms are replaced etc.

FIG. 2C is a partial top plan view of the internal lamellar means 20 inFIG. 2A and shows a modification of the component in FIG. 2B. Theinternal lamellar means 20 is at its internal surface provided with fourshown filling blocks 58 made of rubber, the filling blocks beingprovided for distributing forces that arise during a pressing operationand for supporting a quadrangular forming or working tool.

FIGS. 3A–3E show end views of different variants of presses of pressurecell type according to the invention. The Figures thus show that thesize of the external press body as well as the internal press chamberwith a tray can vary. The tray can be made of different numbers ofinternal lamellar means, and the thickness of the lamellar means canvary. The internal dimension of the tray or load space is suitablywithin the range of 200 mm–2000 mm in diameter. However, it is possibleto provide lamellar means having larger or smaller internal diameter.

FIGS. 3A–3C illustrate some variants where an internal lamellar means 60is arranged between a diaphragm support 62 and a bottom plate 64. Apress plate 66 is arranged above the diaphragm support 62. As theseFigures show, an external press-body-forming lamellar means 68 can bemade in different sizes and have different shapes. The size of theinternal lamellar means 60 can also vary.

FIG. 3D shows another variant having a tray which comprises two lamellarmeans 60 a, 60 b that are arranged between the diaphragm support 62 andthe bottom plate 64.

FIG. 3E shows a variant having a tray which comprises four lamellarmeans 60 a, 60 b, 60 c, 60 d that are arranged between the diaphragmsupport 62 and the bottom plate 64.

FIGS. 4A–4B illustrate alternative geometrical shapes for a tray frameor an internal lamellar means. Thus, FIG. 4A shows an internal lamellarmeans 70 for a tray according to at least one embodiment of theinvention, the lamellar means describing an ellipse and thus encloses anoval space. The lamellar means 70 is provided with turns of a band 72 inthe corresponding manner as previously discussed. FIG. 4B shows analternative internal lamellar means 74 with turns of a band 76. Thisinternal lamellar means has the shape of a super-ellipse, i.e. a closedcurve having the equation |x/a|^(n)+|y/b|^(n)=1, where the exponent n>2.These lamellar means, as well as the ones shown in FIGS. 2B and 2C, arethus circumferentially curved and provided with prestress for enablingindependent force-absorbing function during a pressing operation.

Although some preferred embodiments have been described above, theinvention is not limited thereto. For example, separate lamellar meansand the diaphragm support can be varied in accordance with the currentneeds. In the figures the internal as well as the external surfaces ofthe lamellar means have been shown curved along the entirecircumference. As an alternative to this, the invention also offers thepossibility of having only one of said surfaces of the lamellar meanscurved along the entire circumference. Another alternative is that bothsaid surfaces are curved, but not concentric, i.e. different amount ofmaterial may be present at different portions around a lamellar means.It should thus be understood that a plurality of modifications andvariations can be provided without deviating from the scope of thepresent invention which is defined in the appended claims.

1. A press of pressure cell type, comprising a tray and aforce-absorbing press body, wherein the press body encloses a presschamber, into which a tray is introducible, and the tray comprises atray frame which defines a space for arranging at least one of a formingtool and a workpiece, wherein prestressing means, which induce acompressing prestress which acts in planes parallel to the plane of thetray, are arranged on the external surface of the tray frame, andwherein the tray frame presents a curvature along its entirecircumference.
 2. The press of pressure cell type as claimed in claim 1,wherein the external surface of the tray frame is entirely curved in thecircumferential direction of the tray frame while the internal surfaceof the tray frame, defining said space presents at least one straightportion in the circumferential direction of the tray frame.
 3. The pressof pressure cell type as claimed in claim 1, wherein the internalsurface of the tray frame defining said space is entirely curved in thecircumferential direction of the tray frame, while the external surfaceof the tray frame presents at least one straight portion in thecircumferential direction of the tray frame.
 4. The press of pressurecell type as claimed in claim 1, wherein both the external surface ofthe tray frame and the internal surface of the tray frame defining saidspace are entirely curved in the circumferential direction of the trayframe.
 5. The press of pressure cell type as claimed in claim 1, whereinthe circumference of the tray frame has a geometrical shape chosen fromthe group consisting of circular, elliptical and super-elliptical. 6.The press of pressure cell type as claimed in claim 1, wherein saidprestressing means comprises at least one prestressing element which iswound round the external surface of the tray frame.
 7. The press ofpressure cell type as claimed in claim 1, wherein said tray framecomprises at least one plate-shaped, annular lamella which has a centralhole, a workpiece being adapted to be machined in the space which isformed by the central hole.
 8. The press of pressure cell type asclaimed in claim 1, wherein the tray comprises a number of concentric,plate-shaped, annular lamellas which abut against one another, each havea central through hole and are located in planes that are parallel tothe plane of the tray, a workpiece being adapted to be machined in thespace which is mutually formed by the holes of the concentric lamellas.9. The press of pressure cell type as claimed in claim 6, wherein saidtray frame comprises at least one plate-shaped, annular lamella having acentral hole a workpiece being adapted to be machined in the space whichis formed by the central hole, and said at least one prestressingelement being band-shaped and having substantially the same width as thethickness of lamella, each lamella being provided with a prestressingelement.
 10. The press of pressure cell type as claimed in claim 8,wherein said lamellas are detachable from one another.
 11. The press ofpressure cell type as claimed in claim 7, wherein the lowest lamella isdetachably arranged on a bottom plate in the press chamber.
 12. Thepress of pressure cell type as claimed in claim 7, wherein a diaphragmsupport for holding a diaphragm is arranged above and, when pressing theworkpiece, in abutment against the uppermost lamella in such a mannerthat the diaphragm together with a press plate, which is arranged in theupper portion of the press chamber, forms a pressure cell, the diaphragmin connection with the supply of pressure medium to the pressure cellbeing adapted to exert a forming pressure on the workpiece arrangedbelow.
 13. The press of pressure cell type as claimed in claim 12, whichis designed with such dimensions that at least the diaphragm support,and optionally at least one lamella, is liftable with the purpose ofaccessing the underlying lamellas inside the press chamber, at least oneof said underlying lamellas being removable from the press chamber whilethe diaphragm support and any remaining lamellas are left inside thepress chamber.
 14. The press of pressure cell type as claimed in claim11, wherein the remaining lamellas are removable from the press chamberwhen the press chamber is free from said underlying lamellas.
 15. Thepress of pressure cell type as claimed in claim 1, wherein the tray ismade of hot-rolled steel plate.
 16. The press of a pressure cell type asclaimed in claim 6, wherein the tray comprises a number of concentric,plate-shaped, annular lamellas which abut against one another, each havea central through hole and are located in planes that are parallel tothe plane of the tray, a workpiece being adapted to be machined in thespace which is mutually formed by the holes of the concentric lamellas,and said at least one prestressing element being band-shaped and havingsubstantially the same width as the thickness of a lamella, each lamellabeing provided with a prestressing element.
 17. The press of pressurecell type as claimed in claim 16, wherein said lamellas are detachablefrom one another.
 18. A tray for use in a press of pressure cell type,the tray comprising: a tray frame which defines a space with the purposeof arranging at least one of a forming tool and a workpiece, andprestressing means, which induce a compressing prestress which acts inplanes parallel to the plane of the tray, arranged on the externalsurface of the tray frame, wherein the tray frame presents a curvaturealong its entire circumference.
 19. The tray as claimed in claim 18,wherein the external surface of the tray frame is entirely curved in thecircumferential direction of the tray frame, while the internal surfaceof the tray frame defining said space presents at least one straightportion in the circumferential direction of the tray frame.
 20. The trayas claimed in claim 18, wherein the internal surface of the tray framedefining said space is entirely curved in the circumferential directionof the tray frame, while the external surface of the tray frame presentsat least one straight portion in the circumferential direction of thetray frame.
 21. The tray as claimed in claim 18, wherein both theexternal surface of the tray frame and the internal surface of the trayframe defining said space are entirely curved in the circumferentialdirection of the tray frame.
 22. The tray as claimed in claim 18,wherein the circumference of the tray frame has a geometrical shapechosen from the group consisting of circular, elliptical andsuper-elliptical.
 23. The tray as claimed in claim 18, wherein saidprestressing means comprises at least one prestressing element which iswound round the external surface of the tray frame.
 24. The tray asclaimed in claim 18, wherein said tray frame comprises at least oneplate-shaped, annular lamella which has a central hole, a workpiecebeing adapted to be machined in the space which is formed by the centralhole.
 25. The tray as claimed in claim 18, which comprises a number ofconcentric, plate-shaped, annular lamellas which abut against oneanother, each have a central through hole and are located in planes thatare parallel to the plane of the tray, a workpiece being adapted to bemachined in the space which is mutually formed by the holes of theconcentric lamellas.
 26. The tray as claimed in claim 23, wherein saidtray frame comprises at least one plate-shaped, annular lamella whichhas a central hole, a workpiece being adapted to be machined in thespace which is formed by the central hole, said at least oneprestressing element being band-shaped and having substantially the samewidth as the thickness of a lamella, each lamella being provided with aprestressing element.
 27. The tray as claimed in claim 23, wherein twolamellas which abut against one another are formed in such a manner thata workpiece, which extends transversely to said space, is kept inposition when these two lamellas have been joined.
 28. The tray asclaimed in claim 18, wherein the tray is made of hot-rolled steel plate.29. The tray as claimed in claim 23, which comprises a number ofconcentric, plate-shaped, annular lamellas which abut against oneanother, each have a central through hole and are located in planes thatare parallel to the plane of the tray, a workpiece being adapted to bemachined in the space which is mutually formed by the holes of theconcentric lamellas, said at least one prestressing element beingband-shaped and having substantially the same width as the thickness ofa lamella, each lamella being provided with a prestressing element. 30.A method for manufacturing a tray for use in a press of pressure celltype, comprising the steps of: forming the tray of steel plate, the traycomprising a tray frame, wherein the tray frame is formed such that itpresents a curvature along its entire circumference; and inducing aremaining compressing prestress in the tray, the prestress acting inplanes parallel to the plane of the tray.
 31. The method as claimed inclaim 30, wherein an external surface of the tray frame is formed to beentirely curved in the circumferential direction of the tray frame whilean internal surface of the tray frame, is formed to present at least onestraight portion in the circumferential direction of the tray frame. 32.The method as claimed in claim 30, wherein an internal surface of thetray frame is formed to be entirely curved in the circumferentialdirection of the tray frame while an external surface of the tray frameis formed to present at least one straight portion in thecircumferential direction of the tray frame.
 33. The method as claimedin claim 30, wherein both an external surface of the tray frame and aninternal surface of the tray frame are formed to be entirely curved inthe circumferential direction of the tray frame.
 34. The method asclaimed in claim 30, wherein the circumference of the tray frame isgiven a geometrical shape chosen from the group consisting of circular,elliptical and super-elliptical.
 35. The method as claimed in claim 30,wherein the step of forming the tray comprises the steps of: formingplate-shaped lamellas of steel plate; providing each of said lamellaswith a through hole; and arranging each lamella with the plane of theplate oriented parallel to the plane of the plate of a concentricallyabutting lamellar means, a workpiece being adapted to be machined in thespace which is mutually formed by the holes of the concentric lamellarmeans.
 36. The method as claimed in claim 35, wherein prestressingelements are wound round the external surface of the lamellas with thepurpose of providing said prestressing.
 37. The method as claimed inclaim 36, wherein a prestressing element is used, which is band-shapedand has substantially the same width as the thickness of a lamella. 38.The method as claimed in claim 35, comprising the step of giving thelamellar means the desired shape using a machining step selected fromthe group consisting of milling, cutting, water cutting, plasma cutting,and flame cutting.
 39. The method as claimed in claim 35, comprising thestep of making the lamellas of steel plate having a thickness of 80–200mm.
 40. The method as claimed in claim 35, comprising the step of makingthe lamellas of steel plate having a thickness of 100–150 mm.
 41. Themethod as claimed in claim 35, comprising the step of making thelamellas of steel plate having a thickness of 100–120 mm.